Wireless transmitters are important components in modern communications systems. Wireless transmitters provide modulated radio waves to carry (transmit) data signals from one place to another. Modulation is the process of placing the data signals on the radio wave for transmission. The waves can be frequency modulated (FM, as in FM broadcast radio), amplitude modulated (AM, as in AM broadcast radio), phase modulated (PM), or modulated in other ways known in the art (e.g. frequency shift keying (FSK) or single sideband (SSB)).
Direct conversion wireless transmitters, also known as direct up transmitters, are relatively simple and inexpensive compared to other types of transmitters. Their simplicity and low cost are largely due to the fact that they require fewer components. Direct conversion transmitters generate a transmitted wave with a center frequency (carrier) and small regions of frequencies above and below the carrier (sidebands) that contain the modulated information. In direct conversion transmission, the information resides in the relatively higher power sidebands, and the carrier ends up with little power. This increases the transmission power efficiency and makes a battery last longer.
Direct conversion transmitters have local oscillators, modulating signals, mixers, summers, and amplifiers. The local oscillators generate radio frequency (RF) signals at the transmitter output frequency. The modulating signals contain the information to be transmitted. The mixers electrically combine the modulating signals with the local oscillator signals to create a modulated RF signal. Amplifiers increase the amplitude or strength of signals. Low frequency amplifiers are used to amplify the modulating signals, while high frequency RF amplifiers are needed to amplify modulated radio signals. Summing circuits add two signals to generate a sum signal.
The local oscillators create signals that are at the carrier frequency. During the direct conversion modulation process, the carrier power is reduced to a minimal level, that is the carrier is suppressed. All that remains are the information carrying sidebands. While theoretically it is possible to completely eliminate the carrier power, it is not easy to do with real world local oscillator circuits. In fact, it is quite difficult to suppress the carrier because of various local oscillator circuit errors. These errors change over time and temperature. The local oscillator errors cause the transmitter to emit power at the carrier frequency. The undesired power at the carrier frequency is called local oscillator leakage and reduces the performance of the transmitter. Accordingly, there is a need for direct up circuit topologies that can further reduce and minimize local oscillator leakage.